In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
There are two modes by which the operating mechanism for the circuit breaker can disengage the operating contacts: the circuit breaker operating handle can be used to activate the operating mechanism; or a tripping mechanism, responsive to unacceptable levels of current carried by the circuit breaker, can be used to activate the operating mechanism. For many circuit breakers, the operating handle is coupled to the operating mechanism such that when the tripping mechanism activates the operating mechanism to separate the contacts, the operating handle moves to a fault or tripped position.
To engage the operating contacts of the circuit breaker, the circuit breaker operating handle is used to activate the operating mechanism such that the movable contact(s) engage the stationary contact(s). A motor coupled to the circuit breaker operating handle can also be used to engage or disengage the operating contacts. The motor can be remotely operated.
A typical industrial circuit breaker will have a continuous current rating ranging from as low as 15 amps to as high as 160 amps. The tripping mechanism for the breaker usually consists of a thermal overload release and a magnetic short circuit release. The thermal overload release operates by means of a bi-metalic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as for the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross-section and related elements are specifically selected for such current range resulting in a number of different circuit breakers for each current range.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker. The prior art provides several methods to tune the magnetic trip unit for different trip currents. First, the armature spring force can be varied, by an adjustment or by changing springs, to change the resisting force on the armature, which changes the current required to trip the breaker. Second, the cross section of the steel in either the yoke, armature or both can be adjusted to increase or decrease the amount of magnetic flux created by the short circuit current. One approach to resolving these issues, is to vary the material thickness, i.e., steel cross section of the magnetic trip elements. However, if the magnetic yoke is made thicker for all ratings, then this reduces the space available inside the magnetic yoke. Reduced space means less cross sectional area available for carrying current in the conductors and also less room for making calibration adjustments. Changing the steel thickness also has the disadvantage of changing the features which mount the yoke and armature in the breaker and thus common mount features cannot be used.
Thus, there is a need for a molded case circuit breaker capable of a broad rating application with a system of parts that works throughout a broad range of current ratings, with a minimum of unique parts and manufacturing tools. Further there is a need for a molded case circuit breaker that is compact in size but yet capable of a broad range of current ratings. There is also a need for a molded case circuit breaker that can be easily reconfigured over a broad range of current ratings by utilizing interchangeable parts and additional parts within the tripping mechanism with a minimum of unique parts.